This invention relates to an electrostatographic printing machine, and more particularly concerns an improved development system for use therein.
An electrostatographic printing process forms an electrostatic latent image and reproduces the image in viewable form on a copy sheet. The field of electrostatography includes electrophotography and electrography. Electrophotography employs a photosensitive medium to form, with the aid of electromagnetic radiation, the electrostatic latent image. Electrography utilizes an insulating medium to form, without the aid of electromagnetic radiation the electrostatic latent image. In both of the foregoing processes, the latent image is rendered viewable by the process of development, i.e. depositing particles thereon. Frequently, the particles are transferred from the latent image to a copy sheet, or, in some processes, the recording sheet on which the latent image is produced, may serve also as the copy sheet after the particles have been deposited thereon. In either of the foregoing cases, the resultant toner powder image deposited on the sheet is permanently affixed applying heat and/or pressure thereto. Hereinafter, an electrophotographic printing machine will be described as an illustrative embodiment of these processes.
In electrophotographic printing, the photoconductive member is charged to sensitize its surface. The charged photoconductive member is exposed to a light image of the original document being reproduced. Exposure of the sensitized photoconductive surface discharges the charge selectively in the irradiated areas. This creates an electrostatic latent image on the photoconductive surface corresponding to the original document being reproduced. Development of the electrostatic latent image recorded on the photoconductive surface isachieved by bringing developer material into contact therewith. Typical developer material comprises dyed or colored heat settable plastic powders, known in the art as toner particles, which are mixed with coarser carrier granules, such as ferro-magnetic granules. The toner particles and carrier granules are selected such that the toner particles require the appropriate charge relative to the electrostatic latent image recorded on the photoconductive surface. Thus, when the developer material is brought into contact with the latent image recorded on the photoconductive surface, the greater attractive force thereof causes the toner particles to transfer from the carrier granules and adhere to the electrostatic latent image. This concept was originally disclosed by Carlson in U.S. Pat. No. 2,297,691 and is further amplified and described by many related patents in the art.
With the advent of multi-color electrophotographic printing, it became highly desirably to reproduce color originals as color copies, or even black and white originals as color copies. Heretofore, the process of color electrophotographic printing required the utilization of filters to form successive single color light images from the colored original document. These single color light images record successive single color electrostatic latent images on the photoconductive surface. Each single color electrostatic latent image is developed with toner particles of a color complementary to the color of the filtered light image. These toner powder images, each of a different color, are transferred to the copy sheet in superimposed registration with one another. Thereafter, the multi-layer toner powder image is permanently affixed to the copy sheet. This produces a multi-color copy from a colored original document. The foregoing is fully described in U.S. Pat. No. 3,854,449 issued to Davidson in 1974.
Recently, it has been highly desirable to create a copy containing information in two or more colors. This may be achieved in high speed electrophotographic printing machines by masking selected portions of the original document or utilizing two or more original documents and reproducing the information contained therein on a common copy sheet in different colors. Different colors in the electrophotographic printing machine may be achieved by changing the color of the developer material employed in the development system. Thus, a black and white electrophotographic printing machine may be converted to a color printing machine simply by changing the color of the developer material contained within the development system. This requires cleaning the chamber storing the developer material so that no residual developer material remains therein prior to the introduction of the differently colored developer material. This is ncessary to prevent the contamination of the new charge of developer material by the residual developer material from the prior charge. Hence, it would be highly desirable to utilize a development apparatus containing a plurality of differently colored developer materials stored therein for subsequent application to the latent image automatically.
Accordingly, it is a primary object of the present invention to improve the multi-color capability of the development apparatus of an electrophotographic printing machine.